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The USGS NED (1-arcsec) dataset is available for the continental US as well as most of Canada. It is based on the NAVD88 vertical datum.

I am interested in using the DEM to orthorectify SAR scenes in Northern Canada using the RPC model. This expects a DEM in the WGS84 vertical datum so I need to do a vertical offset of the DEM. This is possible using gdal, however, from what I can tell, the NAVD88 geoid 2012a is only defined in the continental US (plus Alaska, etc.) and parts of southern Canada.

I've downloaded the *.gtx files that are required by gdal (g2012a_conus.gtx, g2012a_hawaii.gtx, g2012a_alaska.gtx etc.), but Is there an issue with calculating the offsets in Northern Canada? Are the gtx files even defined there?

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Is there an issue with calculating the offsets in Northern Canada? Are the gtx files even defined there?

If you open up the *.gtx files in QGIS, for example, you'll find that they are not defined north of about 58 degrees N. You will not be able to calculate geoid offsets outside of where the gtx file is defined.

I don't know if you could possibly substitute another geoid model, but I wouldn't recommend it.

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